1. Technical Field
The present invention relates to the identification of protective antigens against infestations with Ixodes spp. ticks, gene sequences and encoded proteins for such antigens, related vaccines and methods useful to induce an immune response, which are protective to interfere with infestations by Ixodes spp. ticks.
2. Background
Ticks parasitize wild, domesticated animals and humans and transmit pathogens including fungi, bacteria, viruses and protozoon. Currently, ticks are considered to be second in the world to mosquitoes as vectors of human diseases, but they are considered to be the most important vector of pathogens in North America (Parola and Raoult, 2001). Ixodes spp. are distributed worldwide and act as vectors of human diseases caused by Borrelia burgdorferi (Lyme disease), Anaplasma phagocytophila (human granulocytic ehrlichiosis), Coxiella burnetti (Q fever), Francisella tularensis (tularemia), B. afzelii, B. lusitaniae, B. valaisiana and B. garinii, Rickettsia helvetica, R. japonica and R. australis, Babesia divergens and tick-borne encephalitis (TBE) and Omsk Hemorrhagic fever viruses (Estrada-Peña and Jongejan, 1999; Parola and Raoult, 2001). Throughout eastern and southeastern United States and Canada, I. scapularis (the black legged tick) is the main vector of B. burgdorferi sensu stricto and A. phagocytophila (Estrada-Peña and Jongejan, 1999; Parola and Raoult, 2001).
Control of tick infestations is difficult and often impractical for multi-host ticks such as Ixodes spp. Presently, tick control is effected by integrated pest management in which different control methods are adapted to one area or against one tick species with due consideration to their environmental effects. Recently, development of vaccines against one-host Boophilus spp. has provided new possibilities for the identification of protective antigens for immunization against tick infestations (Willadsen, 1997; Willadsen and Jongejan, 1999; de la Fuente et al., 1999; 2000; de Vos et al., 2001). The recombinant B. microplus BM86 gut antigen included in commercial vaccine formulations TickGARD (Hoechst Animal Health, Australia) and Gavac (Heber Biotec S. A., Havana, Cuba) also confers partial protection against phylogenetically related Hyalomma and Rhipicephalus tick genera (de la Fuente et al., 2000; de Vos et al., 2001). However, immunization with BM86 failed to protect against the more phylogenetically distant Amblyomma spp. (de Vos et al., 2001). These results suggest that using Bm86 or a closely related gene for the production of vaccines against Ixodes spp. or other tick genera phylogenetically distant from Boophilus spp. (Black and Piesman, 1994) could be impractical. Therefore, the screening for novel protective antigens is necessary to identify vaccine candidates against infestations with these tick species of medical and veterinary importance. Control of ticks by vaccination would avoid environmental contamination and selection of drug resistant ticks that result from repeated acaricide application (de la Fuente et al., 1998; Garcia-Garcia et al., 1999). Anti-tick vaccines also allow for inclusion of multiple antigens in order to target a broad range of tick species and for incorporation of pathogen-blocking antigens.
Vaccination with DNA and cDNA molecules has been used to induce a protective immune response against B. microplus and several pathogens in laboratory animals and livestock (De Rose et al., 1999; Drew et al., 1999; van Drunen Littel-van den Hurk et al., 2001; Kofta and Wedrychowicz, 2001). A new technique, expression library immunization (ELI) in combination with sequence analysis provides an alternative approach for identification of potential vaccine antigens based on rapid screening of the expressed genes without prior knowledge of the antigens encoded by cDNA clones. ELI was first reported for Mycoplasma pulmonis (Barry et al., 1995) and since then has been used for unicellular and multicellular pathogens and viruses (Manoutcharian et al., 1998; Alberti et al., 1998; Brayton et al., 1998; Melby et al., 2000; Smooker et al., 2000; Moore et al., 2002; Singh et al., 2002). However, the identification of individual protective clones has not been reported and it is predicted that identification of protective antigens will be more difficult as the complexity of the genome increases.
Although several reports in the literature have demonstrated by ELI that libraries can offer a degree of protection (Barry et al., 1995; Manoutcharian et al., 1998; Alberti et al., 1998; Brayton et al., 1998; Melby et al., 2000; Smooker et al., 2000; Moore et al., 2002; Singh et al., 2002), none have applied ELI to arthropods and particularly to ticks. Several vaccines have been developed to protect humans against Ixodes-transmitted pathogens including TBE virus and B. burgdorferi. However, it is not clear whether these vaccines will protect against all pathogen strains and genotypes. The inclusion of tick immunogens in pathogen-specific vaccines could enhance their protective effect and increase efficacy (Nuttall, 1999). This transmission-blocking approach is supported by evidence that host resistance to ticks provides some protection against tick-borne transmission of viruses and B. burgdorferi (Wikel et al., 1997). Furthermore, vaccination against B. microplus has been demonstrated to contribute to the control of tick-borne diseases (de la Fuente et al., 1998; 1999).